Anthrax

Japanese: 炭疽

(1) Anthrax
Definition/Concept Bacillus anthracis is the causative bacterium of anthrax. It is an aerobic Gram-positive rod-shaped bacterium that forms spores. Anthrax is a zoonotic disease that is originally seen in livestock. It is classified as a Category 4 infectious disease under the Infectious Diseases Control Law.
ClassificationAnthrax is broadly classified into three types: 1) cutaneous anthrax, 2) pulmonary anthrax (inhalation anthrax), and 3) intestinal anthrax. Most cases are cutaneous anthrax, and pulmonary and intestinal anthrax rarely develop. The incubation period is usually 1-7 days, but is greatly influenced by factors such as the amount of bacteria infecting the body, and some reports have stated that the longest incubation period is 60 days.
Cause and etiologyHumans are generally infected through infected livestock, and when they handle an infected animal or its hide, the anthrax bacteria enter through a wound in the skin, causing cutaneous anthrax in many cases.In the case of bioterrorism, in addition to cutaneous anthrax, humans are susceptible to inhaling the dispersed anthrax spores and developing pulmonary anthrax.
Epidemiology, incidence, and statistics: This disease is distributed worldwide. In Japan, an outbreak of anthrax was reported in 1965, but since then it has become an extremely rare disease with approximately one case per year or every few years. The former Soviet Union has a history of conducting research on anthrax as a biological weapon, and in 2001 an incident occurred in the United States where the bacteria was sent through the mail for bioterrorism purposes, resulting in several deaths. The US CDC classifies anthrax as Category A, the most dangerous microorganism that could be used as a biological weapon.
Pathophysiology: Even after Bacillus anthracis invades the body, its capsule makes it resistant to phagocytosis by macrophages and other organisms. When the spores germinate and begin to grow, they produce various toxins that cause bleeding, edema, and necrosis. The disease progresses quickly and can easily become severe.
Clinical manifestations
1) Cutaneous anthrax:
The raised lesions that develop at the site of infection turn into painless pustules, and eventually the center undergoes necrosis. The lesions then form dark brown crusts that resemble charcoal, creating the lesions characteristic of cutaneous anthrax. In addition, lymphadenitis in the area surrounding the site of infection is likely to occur (Figure 4-5-5).
2) Inhalational anthrax:
Symptoms begin with cold-like symptoms such as slight fever and fatigue, followed by headache, muscle pain, chills, and chest pain. In many cases, symptoms include dyspnea, cyanosis, and pleural effusion, and the condition progresses rapidly to a state of shock.
3) Gastrointestinal anthrax:
After ingesting contaminated food, patients experience symptoms such as nausea, vomiting, abdominal pain, and fever. Patients may also complain of hematemesis, bloody stools, and diarrhea.
Laboratory findings: Peripheral blood leukocyte counts show an increase in neutrophils. Inhalation anthrax is characterized by mediastinal expansion accompanied by severe lymphadenopathy on chest X-ray, and may also be accompanied by pleural effusion, pulmonary edema, and pulmonary hemorrhage.
Diagnosis Cutaneous anthrax can be suspected when characteristic skin lesions are observed in people who work with livestock. If Gram staining of a sample taken from the lesion shows chains of large rod-like bacteria, the possibility of anthrax increases. Early diagnosis of pulmonary anthrax and cutaneous anthrax is difficult, but since severe anthrax is likely to be accompanied by bacteremia or septicemia due to multiple bacteria, blood cultures and direct smears of peripheral blood are stained and observed. Unlike other Bacillus species, Bacillus anthracis does not exhibit motility because it does not have flagella, and furthermore, it does not exhibit hemolysis when cultured on blood agar medium.
Differential diagnosis of pulmonary anthrax includes fulminant pneumonia caused by other pathogens. However, pulmonary anthrax is primarily characterized by inflammation of the mediastinum, and it is rare for the disease to present with a typical pneumonia appearance. It is important to differentiate intestinal anthrax from intestinal infections and enteritis accompanied by bloody stools.
Complications Anthrax meningitis occurs suddenly within a few days of the onset of anthrax, with meningitis symptoms, sudden loss of consciousness, and a high rate of death.
Course and prognosis: In the case of cutaneous anthrax, the prognosis is good if appropriate treatment is given. However, in the case of intestinal anthrax and pulmonary anthrax, the disease progresses rapidly and the patient is likely to fall into a critical condition, with a poor prognosis.
Treatment, prevention, and rehabilitationAnthrax bacteria are naturally highly susceptible to many antibiotics, including penicillins, carbapenems, quinolones, and tetracyclines. However, in the case of bioterrorism, bacteria that have acquired resistance to penicillin and other drugs may be used. In order to increase the survival rate in pulmonary anthrax, quinolone or tetracycline antibiotics are administered in combination with other drugs in large doses at an early stage before a diagnosis is made. As the disease progresses, patients are prone to dehydration, respiratory failure, and shock, so treatment including whole-body management such as fluid replacement, oxygen inhalation, and vasopressors is also necessary. If the possibility of inhaling anthrax bacteria cannot be denied, preventive oral administration is given for 60 days based on the incubation period. Although anthrax vaccines are used in some areas, there are problems with their effectiveness and side effects, and they are not generally administered. [Matsumoto Tetsuya]
■ References
CDC: Update: Investigation of Bioterrorism-Related Anthrax and Interim Guidelines for Clinical Evaluation of Persons with Possible Anthrax. MMWR, 50: 941-947, 2001.
WHO: Guidelines for the surveillance and control of anthrax in humans and animals. http://www.who.int/csr/resources/publications/anthrax/WHO_EMC_ZDI_98_6/en/

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

（1）炭疽（anthrax）
定義・概念
　炭疽菌（Bacillus anthracis）は炭疽の起炎菌である．好気性のGram陽性桿菌で，芽胞を形成する．炭疽は人畜共通感染症で，本来は家畜にみられる疾患である．感染症法で四類感染症に分類されている．
分類
　炭疽は，①皮膚炭疽，②肺炭疽（吸入炭疽），および③腸炭疽の３種類に大きく分類される．その多くは皮膚炭疽であり，肺炭疽および腸炭疽を発症することはまれである．潜伏期は通常1～7日とされているが，感染菌量などに大きく影響され，最長の潜伏期は60日という報告もある．
原因・病因
　ヒトは一般的に感染した家畜を介して感染するため，炭疽に罹患した動物やその皮革を扱って皮膚の傷から炭疽菌が侵入し，皮膚炭疽を発症することが多い．バイオテロの場合は皮膚炭疽以外に，散布された炭疽菌の芽胞をヒトが吸入して肺炭疽を発症しやすい．
疫学・発生率・統計的事項
　本疾患は世界的に分布がみられる．わが国では1965年に炭疽の集団発生がみられたが，それ以降は年間あるいは数年に１例程度のきわめてまれな疾患となっている．旧ソビエト連邦が生物兵器として炭疽菌の研究を行っていた歴史があり，米国では2001年にバイオテロ目的で本菌を郵便物で送る事件が発生し数名が死亡した．米国CDCは，生物兵器に使用される可能性のある微生物の中で，炭疽菌を最も危険度の高いカテゴリーAに分類している．
病態生理
　炭疽菌は生体内に侵入しても，その莢膜によりマクロファージなどの貪食に抵抗性を示す．また芽胞が発芽して増殖を始めると種々の毒素を産生し，それによって出血，浮腫，および壊死などを引き起こす．病状の進行は速やかであり重篤化しやすい．
臨床症状
１）皮膚炭疽（cutaneous anthrax）：
　感染部位にできた隆起性病変は無痛性の膿胞となり，やがて中央部は壊死を起こす．さらに病変部は外見上，炭のように黒褐色の痂皮を形成し，皮膚炭疽に特徴的な病変ができる．さらに感染部位の所属リンパ節炎を合併しやすい（図4-5-5）．
2）肺炭疽（inhalational anthrax）：
最初は微熱，倦怠感などの感冒様症状から始まり，さらに頭痛，筋肉痛，悪寒，および胸痛を訴える．その後，多くの例で呼吸困難，チアノーゼ，胸水などを伴い，さらにショック状態へと急激に進展する．
3）腸炭疽（gastrointestinal anthrax）：
汚染された食品などを摂取後に悪心，嘔吐，腹痛，発熱などで発症する．さらに吐血，血便や下痢を訴える．
検査成績
　末梢血白血球は好中球優位の増加を示す．肺炭疽では胸部X線で高度なリンパ節腫脹を伴う縦隔の拡大が特徴的であり，さらに胸水貯留や肺水腫，および肺出血を伴うことがある．
診断
　家畜を扱う職業に従事している人などに特徴的な皮膚病変を認めた場合は，皮膚炭疽の推定が可能である．病変部位から採取した検体のGram染色で，竿をつないだような大型の桿菌が連鎖状に観察されれば炭疽の可能性が高くなる．肺炭疽や皮膚炭疽は早期の診断は困難であるが，重症の炭疽では多数の菌による菌血症あるいは敗血症を伴いやすいので，血液培養とともに末梢血の直接塗抹標本をGram染色し観察する．炭疽菌はほかのバチルス属の菌と異なり，鞭毛がないために運動性を示さず，さらに血液寒天培地で培養しても溶血性を示さない．
鑑別診断
　肺炭疽の場合はほかの病原体による激症型の肺炎が鑑別診断にあげられる．ただし肺炭疽では縦隔の炎症が主体であり，典型的な肺炎像を呈して発症することはまれである．腸炭疽は血便を伴う腸管感染症や腸炎などとの鑑別が重要である．
合併症
　炭疽菌性髄膜炎は炭疽発症から数日以内に突然，髄膜炎症状で発症し，急激な意識障害が起こり高い頻度で死亡する．
経過・予後
　皮膚炭疽の場合，適切な治療をすれば予後は良好である．しかし腸炭疽や肺炭疽では急激に病状が進展し重篤な状態に陥りやすく予後は不良である．
治療・予防・リハビリテーション
　炭疽菌は本来，ペニシリン系，カルバペネム系，キノロン系，テトラサイクリン系など多くの抗菌薬に良好な感受性を示す．ただしバイオテロではペニシリンなどへの耐性が付加された菌を使用される可能性がある．肺炭疽では救命率を上げるため，診断が確定する前の早期の段階からキノロン系やテトラサイクリン系抗菌薬とほかの薬剤を併用し大量に投与する．また病状の進行に伴って，脱水，呼吸不全，ショックなどに陥りやすいため，補液，酸素吸入，昇圧剤など全身管理を含めた治療も必要である．なお炭疽菌を吸入した可能性が否定できない場合は，潜伏期間に基づいて60日間の予防内服を行う．炭疽菌のワクチンは一部では使用されているものの有効性や副作用の問題があり，一般的な接種は行われていない．［松本哲哉］
■文献
CDC: Update: Investigation of Bioterrorism-Related Anthrax and Interim Guidelines for Clinical Evaluation of Persons with Possible Anthrax. MMWR, 50: 941-947, 2001.
WHO: Guidelines for the surveillance and control of anthrax in humans and animals. http://www.who.int/csr/resources/publications/anthrax/WHO_EMC_ZDI_98_6/en/

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